When should I start to worry? I need help!

Keri
I got off BC June 12th after being on it for 8 years. I had a "withdra" period June 16-20th. My period is 8 days late today but all of the 8 pregnancy tests I've taken are negative. I know it's normal to go without a period after discontinuing BC but when should I start to worry? I haven't had a period in over a month.